Remove Netty 5 support

See gh-34345

build.gradle

@@ -21,13 +21,6 @@ configure(allprojects) { project ->
 	group = "org.springframework"
 	repositories {
 		mavenCentral()
-		maven {
-			url = "https://repo.spring.io/milestone"
-			content {
-				// Netty 5 optional support
-				includeGroup 'io.projectreactor.netty'
-			}
-		}
 		if (version.contains('-')) {
 			maven { url = "https://repo.spring.io/milestone" }
 		}